1. Technical Field of the Invention
The present invention relates to electric circuit devices for electric circuit substrates on which electric circuit elements are mounted and, more particularly, to an electric circuit device having a case filled with gel to seal an electric circuit substrate and associated electric circuit elements for performing insulation, waterproof and vibrational relaxation of the electric circuit elements.
2. Description of the Related Art
In the related art, an attempt has heretofore been made to provide an electric circuit device having a case filled with gel to seal an electric circuit substrate and associated electric circuit elements for performing insulation, waterproof and vibrational relaxation of the electric circuit elements. Under a circumstance where such an electric circuit device is installed on an internal combustion engine of, for instance, a vehicle, it follows that the electric circuit device operates under severe environments. Especially, under a situation where the electric circuit device is installed in an engine room, the electric circuit device is subjected to a severe heat environment. Moreover, in a case where the electric circuit device is installed on an engine body, it follows that the electric circuit device operates under an extremely severe heat environment.
Under such environments, the electric circuit substrate or the associated electric circuit elements vibrate at increased rates. This causes gel, filled inside the case, to be softened (liquefied), resulting in a functional degradation in insulation, waterproof and vibrational relaxation of the associated component parts. Under such circumstances, a short-circuiting is caused to occur in somewhere in the electric circuit or moisture intrudes to an area around connecting parts. This results in the occurrence of corrosion and disconnection in relevant conducting parts. This causes an issue to arise with the resultant increase in the vibrations of associated component parts and disconnections in bonding areas.
To address the above issues, an attempt has heretofore been made to fill the case with sealants in two layers with a view to preventing damage to internal elements caused by external vibrations, that is, especially, the disconnection of a bonding area (see, for instance, Japanese Patent Application Publication 9-246430). With such a sealant structure, a lower layer gel is covered with resin formed as an upper layer. The upper layer is structured with hard resin with a complex elastic modulus of 0.3 MPa for thereby preventing the vibrations of electric circuit elements or electric circuit substrate.
However, with the upper layer made of resin with such a particular complex elastic modulus, the upper layer bears cracking under cold environment. This causes a drop in vibrational relaxation effect of the upper layer after the occurrence of cracking. This results in a difficulty of preventing the softening of gel in the lower layer and no issue can be addressed. In addition, since the sealant is composed of the upper layer, made of resin, and the lower layer made of gel, a less bonding effect exists between the upper layer and the lower layer is weak. This causes a separation to occur on a boundary layer between these two layers and the vibrational relaxation decreases, resulting in a difficulty of addressing the above issues.